The present invention relates to a source driver, an electro-optical device, a projection-type display device, an electronic instrument, and the like.
As a liquid crystal panel (electro-optical device) used for electronic instruments such as a portable telephone or a projection-type display device, an active matrix type liquid crystal panel using a switching device such as a thin film transistor (hereinafter abbreviated as “TFT”) have been known.
It has been considered that it is difficult to reduce power consumption when employing the active matrix type liquid crystal panel for portable electronic instrument such as a portable telephone. In recent years, the power consumption of the active matrix type liquid crystal panel has been sufficiently reduced. The active matrix type liquid crystal panel has attracted attention in that the active matrix type liquid crystal panel is suitable for increasing the number of colors and displaying a video image.
In general, gamma correction is performed for a drive signal of a display device corresponding to the grayscale characteristics of the display device in order to achieve an accurate image display. Taking a liquid crystal device as an example, a grayscale voltage which is gamma-corrected so that an optimum pixel transmissivity is implemented is output based on grayscale data for grayscale display. A source line is driven based on the grayscale voltage.
In recent years, an increase in image quality of a display image has been increasingly desired. Therefore, an increase in the number of grayscales has been desired for a source driver which drives a source line of an electro-optical device. In this case, it is necessary to supply a larger number of types of grayscale voltages to each output buffer which drives each source line of the electro-optical device.
The screen size and the definition of a liquid crystal panel have been increased. Therefore, the number of pixels (number of dots) per scan line has been significantly increased. This makes it necessary to apply a grayscale voltage selected from a plurality of grayscale voltages to each pixel within one horizontal scan period.
However, since the horizontal scan period has been increasingly reduced, it is difficult to apply a voltage at a desired potential to each pixel within a specified period of time. Therefore, it is difficult to produce a source driver which implements high grayscale accuracy.
In a liquid crystal panel, the voltage supplied to the source line is changed in a given cycle by polarity inversion drive in order to prevent a situation in which a direct-current component is applied to the pixel (liquid crystal) for long period of period. The period of time required for the voltage level to become stable (converge) increases as the change in voltage increases. This further makes it difficult to achieve high grayscale accuracy.
JP-A-7-306660 discloses technology in which stepwise voltages are generated in order to reduce the number of grayscale voltage signal lines, and a pulse width modulation signal is generated by sampling a desired voltage from a plurality of stepwise voltages to express halftone. However, grayscale representation is limited to the pulse-width modulation method. Moreover, it is difficult to increase the image quality when a larger number of grayscales is required.
It is also difficult to set the levels of all of the stepwise voltages with high accuracy. Even if the voltage level can be set with high accuracy, the circuit becomes complicated. In particular, it becomes difficult to generate the stepwise voltages of which the levels are set with high accuracy, as disclosed in JP-A-7-306660, as the number of grayscales increases so that the difference in voltage between the grayscales decreases.
A high-definition image display with an increased number of grayscales is also desired for a projection-type display device.